The present invention relates to a method for evaluating an optical fiber, in particular for determining the general type of optical fiber to which a considered optical fibers belongs, to be used in an automatic fiber splicer for automatically selecting correct splicing parameters and also to a method of setting an optical system used in a splicer.
Equipment and methods for aligning and splicing silica based optical fibers has been developed and improved for many years. The most common method for performing an alignment of optical fibers to be spliced with an accuracy better than 0.2 micron and making accurate splice loss estimation has comprised advanced digital image processing of magnified pictures of the splicing position before, at and after the actual splicing of the fibers, these pictures produced by a camera system. The design of a compact optical system, capable of giving a sharp image of the spliced fibers and their cores before, during and after the splicing which in many cases is made by a fusion process, has been a critical task in developing fiber splicing machines having a high performance, see e.g. T. Haibara, M. Matsumoto, T. Tanifuji and M. Tokuda, xe2x80x9cMonitoring method for axis alignment of single-mode fiber and splice loss estimationxe2x80x9d, Optics Letters, Vol 6, No. 4, April 1983, O. Kawata, K. Hoshino, Y. Miyajima, M. Ohnishi and K. Ishihara, xe2x80x9cA splicing and inspection technique for single-mode fibers using direct core monitoringxe2x80x9d, J. Ligthwave Technology, Vol. LT-2, No. 2, April 1984, and T. Katagiri, M. Tachikura and I. Sankawa. xe2x80x9cOptical microscope observation method of a single mode optical fiber core for precise core-axis alignmentxe2x80x9d, J. Ligthwave Technology, Vol. LT-2, No. 3, June 1984.
In a fusion splicer equipped with a digital camera system, the fibers to be spliced or being spliced or having been spliced are conventionally illuminated by a light source, normally a LED, located at a distance behind the fibers as seen from a lens system. The lens system is focused on some point in the fiber claddings or in the fiber cores and a magnified image of the fibers is created on a CCD matrix (Charge Coupled Device). The electric signal from the camera is A/D converted, and the digital picture is processed in a computer system. The measurement data from the pictures are then used for moving fibers to the desired accurate alignment and for estimating the splice loss.
The optical system of a fusion splicer can also be adapted to image the hot fibers during fusion, see e.g. German patent 40 04 909 and Swedish patent application 9002725-1, filed Aug. 24, 1990. The small difference between emissivity of the fiber cladding and the core at high temperatures, such as those of about 2000xc2x0 C. existing in an electric arc used in the fusion process, make it possible to produce a bright image of the core, conventionally located in the middle of the fiber. The visible and near infrared part of the emitted waves from the heated fibers are collected and detected by the camera system. Hot images can be used for real time processing of the fibers during fusion and for making an accurate splice loss estimation after the splice is completed.
It is well known that core/cladding eccentricity, cleave angle, curl, fiber-end contamination and mode field diameter (MFD) mismatch are the main reasons of fusion splicing loss. An MFD mismatch can significantly influence the splice loss in particular in the case where different types of fibers are spliced to each other. To produce splices having a low loss made between fibers having different MFD it is necessary to characterize the types of fibers to be splice and based on the fiber types, select appropriate splice parameters like overlap, fusion heat and fusion time to be used in the splicing procedure, see e.g. W. Zheng, xe2x80x9cReal time control of arc fusion for optical fiber splicingxe2x80x9d, J. Ligthwave Technology, Vol. 11, pp. 548-553, March 1994, and W. Zheng, O. Hultxc3xa9n and Robert Rylander, xe2x80x9cErbium doped fiber splicing and splice loss estimationxe2x80x9d, J. Ligthwave Technology, Vol. 12, pp. 430-435, March 1994. The appropriate choice of these parameters is highly dependent on the core sizes and the refractive index profiles of the fibers and the refractive index differences between the core and cladding. A method for identifying fibers before fusion for automatic selection of the splice parameters is therefore of great importance for making low loss splices of different types of fibers.
In Swedish patent application 9100979-5 for Telefonaktiebolaget L M Ericsson, inventors Ola Hultxc3xa9n and Wenxin Zheng, a method of determining characteristics of optical fibers is disclosed, the method including analyzing images of heated fibers and in particular light intensity profiles along lines perpendicular to the fibers. The general shape of the central peak and especially its width and height are evaluated. A mathematical method using the same basic analyzing process is disclosed in Swedish patent application 9201817-5 for Telefonaktiebolaget L M Ericsson, inventor Wenxin Zheng.
In an automatic fiber splicer the optical system for imaging an optical fiber on some light sensitive area cannot be easily set for different imaging conditions such a for producing a sharp picture of a cold fiber in which the core is visible or in particular the position and the width of the core are detectable or for producing a sharp picture of a heated optical fiber emitting light so that also in this picture the core region is detectable. Such focusing for different imaging conditions is generally made manually by observing the captured images for different focusing conditions, i.e. for different distances between the object, the optical fiber, and the imaging system, primarily the lens system.
It is an object of the invention to provide a reliable method of deciding the type of an optical fiber.
It is another object of the invention to provide a robust, automatic method of setting an optical system for providing images of an optical fiber in which the core of the optical fiber is visible.
In determining the kind or type to which an unknown optical fiber belongs an automatic fiber splicer using fusion-welding is used having movable clamps or retainers for positioning aligning two fibers, electrodes for producing when energized an electric arc, camera devices such as CCD-matrices and light sources producing a background illumination. These devices are all coupled to electronic circuits containing control means (33) and the necessary driver and interface circuits. A portion of the fiber held one of the clamps is imaged on the light sensitive areas of the cameras through high-resolving lens systems, allowing the core of the fiber to be distinguished in the captured image, both when the fiber is cold and when it is heated such as to about or somewhat lower temperatures used in fusion-splicing and then issue sufficient light for capturing images without using any background illumination. From a first picture taken of the fiber in a heated state a first light intensity profile along a line substantially perpendicular to the longitudinal direction of the fiber is determined in an image processing and analysis module. This profile is further analyzed by calculating the derivative of the profile and comparing the derivative to derivatives of light intensity profiles previously determined for a optical fibers of known different kinds or types. A second picture is taken of the cold fiber for which a second light intensity profile can be similarly determined. This profile is then compared to corresponding profiles previously determined for the known optical fibers. The results of the comparing operations are finally evaluated to decide the kind of the tested fiber.
In an automatic fiber splicer a correct automatic focusing for different imaging conditions can be obtained by executing the following steps in a successive order. The distance between the optical fiber and the optical system of the splicer is varied and pictures are taken for different distances. In the pictures taken light intensity profiles are determined as above which are analyzed to find a measure of the apparent diameter of the optical fiber and a measure of the apparent width of the central peak which is normally obtained in such profiles and corresponds to the high intensity region in the center of the fiber and which at least for a high-resolving optical system corresponds to the core of the fiber. The ratio or quotient of these two measures is calculated and compared to a predetermined value. The distance giving a picture in which the ratio of the measure values agrees with the predetermined value or at least deviates as little as possible from that value is taken as the distance giving a correct imaging. It turns out that by measuring on fibers of different types a predetermined value can be determined which produces good pictures of cold pictures from which valuable information of the core such as its diameter can be obtained and a different predetermined value can be determined producing correspondingly good pictures of heated fibers.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.